This invention relates to a stereocontrolled total synthesis of the known antibiotic thienamycin (I). ##STR2## Starting from acetone dicarboxylate, the synthesis proceeds in a stereo-selective way via intermediate II: ##STR3## wherein R is a readily removable protecting group such as benzyl, .beta., .beta., .beta.,-trichloroethyl, methyl, ethyl, phenyl, t-butyl and the like.